The present invention relates generally to combiners, and more particularly, to a combiner having phase and delay correction that permits combining of signals received from a moving satellite using two or more antennas.
When the communication frequencies and the symbol rates are low, the combining of the output of two separate antennas to improve the signal to noise ratio is a problem that was solved many years ago. The solution to this problem is discussed in an article by D. G. Brennan, entitled Linear Diversity Combining Techniques, Proc. of the IRE, Vol. 47, No. 6, pp. 1075-1102, June 1959. The optimal solution is to combine the signals from separate antennas by weighting the signals according to their signal to noise ratio and then add the signals. The signals must be added in phase, so the first adjustment is to correct the phase of the antennas such that all of the signals are in phase.
Referring to the drawing figures, FIG. 1 shows the basic concept of the combining process. The combining process adjusts the gain for each of the antennas depending on the signal to noise ratio at the respective antenna, shifts the phase so that all of the signals are in phase and adds the signals to provide the input to the receiver.
When like antennas are closely spaced such that inter-arrival delays are much less than a received symbol period, the signal to noise ratio is the same at each of the antennas, so the combining process is one that applies a phase shift to each signal such that the signals combine in phase.
When the received signal is from a fixed source, the phase adjustment for combining is constant and can be built into circuits of the combiner. Such a circuit that has a constant bandpass, but a phase shift that depends on the components of the circuit, is well known in the art.
As satellite communication became more established, the possibility of moving sources were taken into account. The phase adjustment of the combiner was made variable by using phase locked loop technology to adjust the phase of oscillators used in downconverters to correct for the phase errors of each of the signals from the antennas.
As the frequencies of communication with satellites increase and the symbol rates increase, the previous solutions are no longer adequate. Antennas that were previously considered to be close together become effectively farther apart as the wavelength decreases and the data rate increases. A primary difficulty is that of aligning signals such that the communication symbols are aligned as well as the phase of the signals.
FIG. 2 illustrates the change in differences in satellite range from two different points on the Earth as a satellite moves. In particular, as the satellite moves, differences in range to the ground terminals changes. A satellite that moves one degree at geosynchronous altitude causes a change in path length at the latitude of the United States of nearly 50 miles. Two antennas that are 10 miles apart may have a difference in change in path length of 700 feet.
The symbol rates for satellite communication may range from hundreds of megasymbols per second to gigasymbols per second. At one gigasymbol the propagation length of one symbol is approximately one foot. If the signals are not aligned to within a delay of a small fraction of a symbol period, not only must the phase be adjusted, but the relative delay must be adjusted before the signals can be combined.
One technique for adjusting the phase and delay of a signal is to sample each of the signals from the antennas and convert the signals to a digital form. The signal samples from the antennas may be pulled from a buffer, time-aligned, interpolating between samples if necessary to establish the correct timing. The phase of a digital signal may be adjusted by multiplying it by a complex phase shift signal. Typically the signals are converted to baseband before sampling, since the sampling rate to prevent aliasing (which must be higher than twice the highest frequency) can be higher than is practical for RF or IF signals.
Once the communication symbol rate increases to hundreds of MHz or GHz, the option of sampling the signals and performing processing digitally is less attractive. Conversion rates are very high, and after conversion the computation rate required to determine the delay adjustment and the phase shift is very large. After the amount of the delay adjustment and phase shift are determined the hardware required to make the delay adjustment and to correct the phase must be very fast.
The size of the problem is established by considering a typical satellite communication problem. A ground station communicating with a satellite in a nominal geosynchronous orbit that is ten degrees away from the longitude of the ground station will see variations in the distance to the satellite that range nearly 50 miles due to small orbital shifts caused by gravity effects of the moon, planets, and solar wind.
The difference in range to the satellite between two antennas that are spaced 10 miles apart can be as much as 6.8 miles. The change in the difference is approximately 700 feet due to satellite motion. It is the difference in distance to the antennas that must be taken into account in combining the antenna signals.
For a signal with a symbol rate of one gigasymbol per second, the propagation length of one symbol is approximately one foot. A change of 700 feet changes the received symbol timing between two antennas by 700 symbols, which is a large number of symbols. At two gigasymbols the difference in received symbol timing varies 1400 symbols, which is a very large number. Even for a symbol rate of 100 megasymbols per second, the difference in the number of symbols between the two antennas varies by 70 symbols. Thus the adjustment of the symbol timing between the antennas must cover 70 to 1400 symbols or more depending on the symbol rate of the communication signal.
There is also a fixed delay that must be taken into account. The signal must be transferred from the antennas to the combining network, introducing a fixed delay that adds to the minimum delay difference between the signals at the antennas. The variable delay must be added to the fixed delays to correct the timing of the antenna signals.
Thus, from the above, when two or more antennas are used to receive a signal from a moving satellite, previous technology is not adequate to combine the signals. Accordingly, it is an objective of the present invention to provide for a combiner having phase and delay correction that permits combining of signals received from a moving satellite using two or more antennas. It is a further objective of the present invention to provide for an improved combiner that employs an optical variable delay.